1. Field of the Invention
This invention relates to a sheet transporting device for transporting a sheet of printing medium, such as regular paper, coated paper, overhead projector sheet, glossy paper, and glossy film. More specifically, such a sheet transporting device may be used for a printer.
Generally, known sheet transporting devices use a pair of rubber rollers, where one of the paired rollers is a drive roller and the other is a follower roller. This arrangement causes a number of problems with sheet transporting accuracy, especially if the drive roller is formed with a rubber roller. First, it is difficult to produce a rubber roller with an accurately dimensioned outside diameter. Second, rubber rollers wear quickly and the thermal expansion of the roller is large. Finally, the coefficient of friction of the roller may be markedly reduced by paper powder, dust and chemicals of coated paper, which attach to the surface of the roller.
A sheet transporting device that attempted to solve the above problems is disclosed in Unexamined Japanese Patent Application No. Hei. 4-140247. In this sheet transporting device, the drive roller is a metal roller with a surface that has been subjected to a blasting process and has a highly accurate outside diameter, and the follower roller is an elastic roller, the surface of which is coated with silicon. According to such a sheet transporting device, because the friction coefficient of the elastic roller is small, the roller pair exhibits a low resistance to paper transportation, and further, the use of the rigid roller blasting process ensures stable transportation of the printing medium.
This sheet transporting device, however, may suffer from other disadvantages. For example, during the blasting process, the raised edges or peaks of the irregular surface of the metal drive roller are easily plastically deformed. As a result, when the printing medium is a film, the coefficient of friction of the paired rollers is insufficient, and transporting accuracy is decreased. Additionally, the paired rollers are easily worn, and, hence, their durability is low.
As such, there is a need for a sheet transporting device that is durable, and accurately and reliably feeds printing media, including media such as films.